Lubricating oils and greases, and similar oleaginous materials are used under circumstances which contribute to their breakdown during normal service. The severe high temperature operating conditions of modern engines accelerate deterioration of lubricants due to oxidation. Oxidative deterioration is accompanied by formation of gum, sludge and acids which may cause metal corrosion as well as chemical breakdown of the lubricant. One of the principal targets of lubricant improvement is their anticorrosive properties with respect to other lubricant additives.
Additives performing as antioxidants, antiwear agents and dispersants are often corrosive themselves and/or break down during normal use into corrosive substances which result in severe corrosive attack on metals and premature replacement of machinery.
In the past, diphenylamines have been used as antioxidants and corrosion inhibitors for various lubricating compositions. Specifically, p,p'-dioctyl-diphenylamine has been used for such application either alone or in combination with other corrosion inhibitors to enhance its corrosion inhibiting properties towards certain metals, particularly copper.
It is known that condensation products of diphenylamine and ketone have been used as antioxidants in various lubricating compositions. However, the individual condensation products of diphenylamine and ketone do not perform satisfactorily at higher operating temperatures, especially in engines. Particularly unsatisfactory protection is obtained against the formation of acids and sludge. The prior art condensation products have been prepared by high temperature reaction methods, e.g. 200.degree. C. which yield products of cyclic structure. It has now been discovered that the corrosion properties of the condensation products can be improved by conducting the condensation reaction at low temperature so that predominately linear condensation products are obtained.